1. Field of the Invention
The present invention relates to a polymer optical waveguide pattern formation method using a polymer material. The present invention can be utilized in various optical waveguides, optical integrated circuits, optical wiring boards and the like which are used in general optical and micro-optical areas and in optical communication or optical information processing areas.
2. Description of the Related Art
Pushed by market requirements and national policy, construction of high-capacity optical fiber networks and preparation of FTTX (fiber to the X point) are being promoted. That is, WDM-MUX/DEMUX (Wavelength Division Multiplexing-Multiplexer/Demultiplexer) using an arrayed waveguide grating (AWG) as a key device has reached a practically applicable level, and a high-capacity and high-expandability network has become available. The demands of the market are expected for changes into optical networks of large-scale nodes, local networks, and various LAN systems in addition to transmission lines and MUX/DEMUX, in the future.
Polymer material is an optically isotropic amorphous material of which optical propagation loss is low as is inorganic glass. Application of polymer material to passive optical circuits is expected to be promising. Further, utilizing its thermo-optic (TO) constant, which is an order of magnitude greater than glass, polymer material has begun to be employed as a waveguide material to fabricate TO switches and the like. Specific waveguide materials can include acrylic polymer, acrylic resin, polyimide, silicone resin, epoxy resin, polycarbonate and the like. Various characteristics are required for waveguide materials. Among them, transparency, heat stability, optical isotropy, and processability are particularly important characteristics.
Most polymer materials are highly transparent in the visible region. On the other hand, overtone of vibration absorption of carbon-hydrogen bond (such as hydrocarbon skeleton) or oxygen-hydrogen bond (such as hydroxy group) causes decrease in transparency in the near infrared region, which is considered as communication wavelength region. Therefore, a fluorocarbonization of the basic skeleton and introduction of siloxane skeleton are being attempted.
A rigid polyimide skeleton, resilient siloxane skeleton, and bridged structure formed by heat or light are being employed to improve heat stability.
A component having optical anisotropy such as aromatic ring should not be oriented in order to improve optical isotropy. However, heat stability and optical isotropy are difficult to realize simultaneously because the rigid or resilient skeleton to improve the heat stability as described above promotes orientation of molecule.
When optical waveguides are fabricated, the processability primarily indicates formability of core-clad layer structure. When a high molecular-weight polymer material is spin-coated from a solution, an intermixing between core and clad layer tends to occur, which is often a problem in waveguide processability. On the other hand, when a low molecular-weight oligomer is spin-coated and then bridged by light or heat, since the bridged polymer film becomes insoluble in a solvent, intermixing can be prevented. As a result, it tends to have a superior processability.
Polymer materials are suitable for producing large-area optical waveguides because they are readily formed into thin films by a spin coating method or a dipping method. Further, according to such a method, since film is not formed at high temperatures, it has an advantage that optical waveguides can be constructed on substrates such as semiconductor substrates or plastic substrates which are difficult to be heat-treated at high temperatures. Still further, it is possible to produce a flexible optical waveguide utilizing the flexibility or tenacity of polymer materials. For such reasons, it is expected to produce optical waveguide parts in large quantity and at low cost by using polymer optical materials: such optical waveguide parts include optical integrated circuits used in the field of optical communications, optical wiring boards used in the field of optical information processing and the like.
Polymer optical materials have been considered to have problems in terms of environmental resistance such as heat stability or moisture resistance. However, a material with heat stability by introducing an aromatic group such as benzene ring and/or an inorganic polymer is disclosed recently, for example, in Japanese Patent Application Laid-open No. 3-43423(1991). Polymer materials have advantageous characteristics in thin film formation and heat treatment process as described above, and problems such as in heat stability or moisture resistance are being improved.
The following methods are reported to form polymer optical waveguides, such as a photo-locking or selective photo-polymerization method (Kurokawa et al., Applied Optics vol. 17, p. 646, 1978) in which a monomer is included in a polymer material, the polymer material is reacted partly with the monomer by irradiation with light to produce a refractive index difference between the irradiated part and unirradiated part; an applied method such as lithography or etching used in semiconductor processing (Imamura et al., Electronics Letters, vol. 27, p. 1342, 1991); and a method using a photosensitive polymer or resist which is sperior in simplicity and mass production adaptability (Trewhella et al., SPIE, vol. 1177, p. 379, 1989). Further, a waveguide production method in which a photopolymerization initiator is added to an epoxy oligomer or the like, and a core is formed by irradiation of light, and then an uncured part is removed, is disclosed in Japanese Patent Application Laid-open No. 10-253845(1998).
As described above, there are many requirements for polymer materials used for optical waveguides. Among them, there are some requirements such as heat stability and optical isotropy. They are based on ideas contrary to each other in the molecular design. Consequently, there is very few material which meets all of such requirements at the same time. However, such a material is not absolutely unavailable, and there is an example of a thermosetting silicone resin: the compatibility between transparency and heat stability can be established by using a ladder siloxane skeleton; optical isotropy can be secured by random thermal crosslinking; and core-clad layer structure formation can be facilitated by thermally crosslinking a film formed with an oligomer and making the film insoluble in solvents.
As described above, silicone resin has superior characteristics as an optical waveguide material, but processability has not been satisfactory. For example, dry etching in the production of a core ridge requires a long time and a plurality of processes, like an inorganic material such as glass or semiconductor. Therefore, as to silicone resin for optical waveguide, as already realized in certain polymers, that is, a photocurable resin, it is desirable that the core-ridge can be directly produced by a simple method in which the resin is photo-crosslinked and an unreacted part is washed out by a solvent.
Normally, as a method for providing a silicone oligomer with a photocurability, a method is used in which epoxy group or vinyl ether group or radical polymerizable acrylic group is introduced into the silicone oligomer itself by covalent bond. However, in these methods, bond between side chains by crosslinking is dominant rather than siloxane bond, which generates problems not only in heat stability but also in an inevitable increase in optical propagation loss due to increase in ratio of hydroxy group in the case of epoxy or due to increase in ratio of hydrocarbon in the case of vinyl ether or acrylics.
In view of such circumstances relating to a polymer material for an optical waveguide, the present invention relates to a photosensitive composition for producing an optical waveguide and a polymer optical waveguide pattern formation method. The present invention provides an optical waveguide material consisting of a polymer material with a photosensitivity. This invention discloses simple and high-speed waveguide formation with outstanding performance in all of transparency, heat stability, optical isotropy and processability.
When an aromatic group such as benzene ring is contained in a molecular structure in order to improve the heat stability, the aromatic group such as benzene ring is oriented to manifest an optical anisotropy thereby increasing birefringence. Because an optical waveguide or the like produced using such a material has a polarization dependence, its output characteristic is changed by variation of polarization plane even when the intensity of incident light is constant. In particular, this is a problem when the waveguide is actually used as a single mode optical waveguide. To eliminate the polarization dependence, it is necessary to use in combination with a polarizer. However, this method has a disadvantage because it makes the construction of the optical device substantially complicated.
The polymer optical waveguide pattern formation method according to the present invention has been made in view of such present circumstances. The object of the present invention is to form a polymer optical waveguide pattern which is simple and suitable for mass production and can be easily connected with optical components by using a reactive oligomer which has simple pattern formation ability, superior heat stability and moisture resistance, small birefringence, and superior processability.
Therefore, in a first aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer, a polymerization initiator and a crosslinking agent, in which the organic oligomer is a silicone oligomer represented by the following formula (1): 
wherein X denotes hydrogen, deuterium, halogen, an alkyl group or an alkoxy group; m is an integer from 1 to 5; x and y designate the proportion of respective units, and neither x nor y is 0; and R1 denotes a methyl, ethyl, or isopropyl group.
Here, the photosensitive composition for optical waveguides may be cationic photopolymerizable, and the crosslinking agent capable of greatly activating the photopolymerizability of the silicone oligomer as a main component of the composition may have at least an epoxy moiety or an alkoxysilane moiety in the molecule.
In a second aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer and a polymerization initiator, in which the organic oligomer is a silicone oligomer represented by the following formula (2): 
wherein X1 and X2 may be the same as or different from each other, and represent hydrogen, deuterium, halogen, an alkyl group or an alkoxy group; m is an integer from 1 to 5; and Z denotes an epoxy group shown in the following formula (I) or (II): 
wherein x and y designate the proportion of respective units; and y is smaller than x and may be 0.
In a third aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer, a polymerization initiator and a crosslinking agent, in which the organic oligomer is a silicone oligomer represented by the following formula (3): 
wherein X denotes hydrogen, deuterium, halogen, an alkyl group or an alkoxy group; m is an integer from 1 to 5; x and y designate the proportion of respective units, and neither x nor y is 0; and R1 and R2 may be the same as or different from each other, and denote a methyl, ethyl, or isopropyl group.
Here, the photosensitive composition for optical waveguides is cationic photopolymerizable, and the crosslinking agent capable of greatly activating the photopolymerizability of said silicone oligomer as a main component of the composition may have at least an epoxy moiety or an alkoxysilane moiety in the molecule.
In a fourth aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer and a polymerization initiator, in which the organic oligomer is a silicone oligomer represented by the following formula (4): 
wherein X denotes hydrogen, deuterium, halogen atom, alkyl or alkoxy group; m is an integer from 1 to 5; x and y designate the proportion of respective units, and neither x nor y is 0; and R1 and R2 may be the same as or different from each other and denote a methyl, ethyl, or isopropyl group.
In a fifth aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer and a polymerization initiator, in which the organic oligomer is a oligomer represented by the following formula (5): 
wherein R1 and R2 may be the same as or different from each other and denote hydrogen, halogen, an alkyl group, an alkoxy group or a trifluoromethyl group; X1, X2 and X3 may be the same as or different from each other, and denote a connection group including at least one selected from the group consisting of an alkylene, alkyleneoxy, oxyalkylene and aromatic group; and Y denotes a polymerization activating group.
In a sixth aspect of the present invention, a photosensitive composition for optical waveguides comprises: an organic oligomer and a polymerization initiator, in which the organic oligomer is a oligomer represented by the following formula (6): 
wherein R1 and R2 may be the same as or different from each other, and denote hydrogen, halogen, an alkyl group, an alkoxy group or a trifluoromethyl group; X1, X2 and X3 maybe the same as or different from each other, and denote a connection group including at least one selected from the group consisting of an alkylene, alkyleneoxy, oxyalkylene and aromatic group and including at least one OH group; and Y denotes a polymerization activating group.
In a seventh aspect of the present invention, a method of producing the above photosensitive composition for optical waveguides comprising the steps of: heating a silicone oligomer and a crosslinking agent in the presence of a solid catalyst; and filtering the solid catalyst, concentrating filtrate, and further adding a polymerization initiator. With this method, initiation of optical crosslinking is remarkably improved.
In a eighth aspect of the present invention, a method of forming a polymer optical waveguide pattern, comprising the steps of: applying and drying the above photosensitive composition for optical waveguides; irradiating said resultant photosensitive composition thin film for optical waveguides with light through a mask; and directly forming a core-ridge pattern by wet etching said photosensitive composition thin film. With this method, a waveguide ridge pattern having a sharp and smooth wall surface can be simply formed without using dry etching or the like.
The inventors have found that these reactive oligomers have a simple pattern formability and are capable of forming a polymer optical waveguide pattern which has superior heat stability and moisture resistance, small birefringence and is easy to connect to optical components, and accomplished the present invention.
Specifically, the present invention is capable of forming a waveguide ridge pattern having a sharp and smooth wall surface by curing a film by irradiation with light and developing by an appropriate solvent. Further, although it has been very difficult to perform waveguide processing in a thick film polymer with material in prior art, the present invention can easily process the waveguide even with a thick film. Still further, with the present invention, birefringence of the photocured of liquid oligomer is reduced to less then 1xc3x9710xe2x88x923, and polarization dependence can be reduced to less than the tolerance limit. Yet further, by controlling the molecular weight of the polymer optical material, an appropriate viscosity suitable for the thin film formation process can be obtained.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.